Happy Valentine Formation Day!

With Valentine's Day having come on Friday, I thought it would be fun to show some love to the Valentine Formation. This geologic unit, from the Middle Miocene of Nebraska, is a great fossil producer but not especially famous, so let's give it a moment in the spotlight.

On fossils in metamorphic rocks

One of the introductory-level bits of information people learn about fossils is that fossils are found in sedimentary rocks, not igneous rocks or metamorphic rocks. This works as a first-order approximation, but... it's not strictly true. For example, among igneous rocks, there are some that overlap sedimentary rocks, e.g., pyroclastic flows, ash beds, and so on. You can certainly find fossils in those. There are also occasional impressions of things in basalt flows; tree trunk molds are most common, and at least one rhino (the famous "Blue Lake rhino"; e.g., Beck 1937) has been captured this way. Tree molds are also interesting for testing our conventional definitions of paleontology in other ways: Because of the rapid cooling and hardening of the basalt, tree molds are essentially instant fossils. The rock can't get much more lithified, after all (especially compared with Quaternary sedimentary slop), yet modern examples haven't put in the time that frequently quoted definitions insist on for fossils, such as 10,000 years. In my day job we talk about paleontological resources as evidence of life in a geological context, so there is no cutoff. Where was I? Oh, yes. Basically, you can get fossils in igneous rock if the rock was playing by sedimentary rules when it formed. Don't expect them in intrusive igneous rocks, though, except in xenoliths of sedimentary host rock (e.g., McCracken et al. 2000, also featuring cores!).

For metamorphic rocks, most of us with paleontological experience will make exceptions for low-grade metamorphics: quartzites, slates, low-grade marbles, things like those. In fact, a lot of rocks commonly called "marbles" are just limestones that can hold a polish. Get much beyond that, and surely the temperature, pressure, chemically active fluids, and mineralogical changes will destroy any unsuspecting crinoid or brachiopod, right?

Well...

Obviously, the answer is "not always", otherwise there wouldn't be a post.

Reports of fossils in metamorphic rocks even as far gone as eclogite (Cavrgna-Sani et al. 2010) or migmatite (Hill 1985; the rock you get when it stops screwing around with metamorphism and gets down to serious melting, but doesn't finish) pop up every so often in the literature as a thin thread of geological quasi-Forteana. The "thin thread" is because they are not common. If they *were* common, they wouldn't be worthy of comment. You don't get a paper out of announcing that there are crinoid columnals in the Decorah Shale. Bucher (1953) is perhaps the most significant reference on the subject, and includes a good bibliography, but through no fault of its own is now more than 70 years old and likely to only get older. Labora-López et al. (2015) adds a few more recent references in its discussion, although it has its own purposes that do not include being a thorough review and update of Bucher (1953).

Aside from being a curiosity, fossils in metamorphic rocks are useful for providing age controls that are notoriously absent from many metamorphic units, and sometimes can help with correlations. Other interpretations are limited by distortion or general poor preservation of the fossils, and by the topic of Labora-López et al. (2015), the loss of information from the sedimentary matrix itself.

One interesting aspect evident from Bucher's lists is that the fossils are not necessarily confined to animals with hard parts that are particularly resistant to replacement (e.g., crinoids). There are recrystallized bivalves, for example. Of course, bivalve replacement is hardly unusual in itself, as shells of bivalves and other mollusks are prone to replacement anyway thanks to the poor stability of aragonite over geologic time, which is why their fossils are so often molds and casts. Some fossils have been replaced by more exotic materials, including echinoderm ossicles replaced by diopside and epidote in a contact metamorphism zone.

Why have fossils in metamorphic rocks not attracted much attention? A not-insignificant part of the problem is people haven't generally been looking for them. As noted by Bucher (1953), paleontologists weren't looking because they assumed that metamorphism destroyed fossils, metamorphic rocks are a pain in the neck to work with compared to sedimentary rocks, and the fossils that have been found in metamorphic rocks are not of great quality, so why bother? Meanwhile, the hard-rock specialists weren't looking because they were hard-rock specialists and weren't interested in fossils.

Still, though, even if this means that fossils are actually some order of magnitude more abundant in metamorphic rocks than generally thought, they aren't exactly falling out of schists and gneisses. Bucher had an elegant proposal: he suggested that most metamorphic rocks of appropriate age to have fossils of things like brachiopods, crinoids, and mollusks are derived from rocks that were deposited in settings with few fossils to begin with. The exact language used is more complicated and bound up in pre-plate-tectonic thought and geosynclines (well, it was 1953, after all), but you get the point: It's hard to have metamorphosed fossils if you don't have fossils to start with. Another rule of thumb Bucher proposed is that fossils can persist if metamorphism does not involve mechanical effects and the fossils are much larger than the size of newly formed minerals. The chances aren't great, but if you're face to face with a metamorphic rock that began as a sedimentary rock of Phanerozoic age, there might just be a stubborn brachiopod or crinoid in there.

References

Beck, G. F. 1937. Remarkable west American fossil, the Blue Lake Rhino. The Mineralogist 5(8): 7–8,
20–21.

Bucher, W. H. 1953. Fossils in metamorphic rocks: a review. Bulletin of the Geological Society of America 64: 275–300.

Cavargna-Sani, M., J. L. Epard, and W. L. Taylor. 1997. Discovery of fossils in the Adula nappe, new stratigraphic data and tectonic consequences (Central Alps). Bulletin de la Société vaudoise des Sciences Naturelles 92: 77–84.

Hill, M. L. 1985. Remarkable fossil locality: crinoid stems from migmatite of the Coast plutonic complex, British Columbia. Geology 13: 825–826.

Laborda-López, C., J. Aguirre, and S. K. Donovan. 2015. Surviving metamorphism: taphonomy of fossil assemblages in marble and calc-silicate schist. PALAIOS 30: 668–679.

McCracken, A. D., D. K. Armstrong, and T. E. Bolton. 2000. Conodonts and corals in kimberlite xenoliths confirm a Devonian seaway in central Ontario and Quebec. Canadian Journal of Earth Sciences 37(12): 1651–1663.

Journey Somewhat Nearer to the Center of the Earth: fossils in cores

Most of the time, when people are looking for fossils, they find them at the surface or just below. However, this is hardly the limit of where they can be found. After all, a fossiliferous formation found at the surface in one location may be buried hundreds to thousands of feet beneath other rocks and sediments somewhere else, and it doesn't stop being fossiliferous just because it's buried that far down. It just becomes much less accessible. We can get glimpses of the buried fossils through core samples.

Prosauropod rehab

I once wrote that "When you get interested in something, what's already there when you arrive will seem like it has always been there, and everything that comes along later will always seem a little new to you, even if it has been decades." Taxonomic fashion is one of those things. When I got into dinosaurs in the mid-1980s, prosauropods had shrunk to something like a half-dozen well-regarded genera plus some dross and a few names that were too new for someone to have gotten around to sinking. One of the survivors was always Ammosaurus, which somehow parlayed a few pelvic differences from Anchisaurus into family-level separation (Ammosaurus the plateosaurid, Anchisaurus the anchisaurid). Meanwhile, all of Africa's prosauropod diversity was being crammed, with varying degrees of success, into Euskelosaurus, Massospondylus, these things called "roccosaurids", and Aristosaurus. (Aristosaurus? You had to be there.) Since I had no personal memory of the work that had gone on in years before, and at five years old wasn't really pouring through the primary literature, it seemed like things had always been that way. This is not the case, though. The vast majority of this synonymization had taken place in the previous 10–15 years and was the work of just a few authors. What had come before that?

Well, obviously, what had come before that were the decades of describing and naming scads of prosauropods; otherwise, there wouldn't have been anything to synonymize. It seems like people loved to name them and then forget about them. The history of their research is littered with misconceptions, some of which seem blindingly obvious in hindsight (big predatory prosauropods? You sure you're not just looking at shed crowns from carnivorous animals?). It's a matter of lack of respect and interest: If dinosaurs were a sideshow to the evolution of important things (i.e., mammals), prosauropods were a sideshow to the sideshow, a couple of card tables featuring a guy with a big unibrow and someone doing "pull my finger" jokes.

Skip ahead a few decades. Some of my readers will have never heard of Aristosaurus (well, okay, nobody living has heard of Aristosaurus except me and maybe 10 other people who have been sworn to secrecy, and now I've lost my membership in the group for a cheap gag) and know only of Euskelosaurus as this thing that nobody talks about except to badmouth in the "Background" section of papers. You wouldn't remember the heady days of the '00s, when that genus collapsed under the weight of its synonyms. Instead, it will have always been a nomen dubium to you, and if it somehow returned (stranger things have happened), it wouldn't be a resumption of your interrupted Euskelosaurus service but something new. (Why, yes, I do have a sense I'm getting older. The only thing that saved me from being older than all MLB players this year was four games and 3 2/3rds innings from Rich Hill.)

Anyway, I've long had a soft spot for prosauropods. They're never been supremely popular and had the poor marketing insight to go extinct before they could be menaced by particularly charismatic theropods, but they have a subtle charm, like small bipedal ornithischians. Over the past year or so, there's been a series of papers providing redescriptions and historical analyses of some of the most venerable genera and species of prosauropods, including comments on some of those 1970s–1980s synonymizations. They include Barrett and Chapelle (2024) on Massospondylus, Barrett and Choiniere (2024) on Melanorosaurus, and Regalado Fernández et al. (2023) and Schaeffer (2024) on Plateosaurus.

Massospondylus

Another instance of perception not matching up with chronology: one of the things that comes out of Barrett and Chapelle (2024) was that before the 1970s, Massospondylus was kind of a dog of a taxon. It achieved its present exalted status mostly by getting there first and being less of a dog than the other choices. If Richard Owen had decided to lead with Leptospondylus or Pachyspondylus in 1854 instead of Massospondylus, it wouldn't have had that first point in its favor, and then where would we be? When a name was needed to absorb the mid-sized prosauropods of the Early Jurassic of southern Africa, M. carinatus was there waiting. Some of the new material was good enough to support a name, and our conception of Massospondylus carinatus drifted from the varied bones named by Owen to that material, which eventually supplied a neotype (a type or name-bearing specimen selected after a species was named, to replace a type that was lost or not specified). The original material was no longer available to query, having been destroyed in World War II, but casts and line drawings of some of it remain, and Barrett and Chapelle (2024) provided redescriptions of as much as they could. In the end, it's just as well that the species is now represented by a neotype, because that original material is not diagnostic beyond the level of Massospondylidae at best. The drift of the conception of Massospondylus, plus the uncritical assignment of a lot of material since the 1970s, leaves the possibility of surprises once "Massospondylus" specimens are thoroughly examined. This also applies to Melanorosaurus and Plateosaurus. (Granted, I'm not convinced that every new genus and species will stand the test of time, but that's for another time, when the pendulum eventually swings back to consolidation.)

Massospondylus as a flowchart. Figure 4 of Barrett and Chapelle (2024). CC BY 4.0.

Melanorosaurus

Proposed synonymizations don't always pan out, of course (one reason not to go overboard committing to the conclusions of New Paper Of The Week). The proposed sinking of Lufengosaurus and Yunnanosaurus into Massospondylus at the height of prosauropod consolidation did not attract much support, and the proposed sinking of Melanorosaurus into Euskelosaurus fared only marginally better. Although it escaped that fate, Melanorosaurus has never seemed to get much respect. It's almost a reflex for authors to mention its questionable type material, as if apologizing for trotting it out. Barrett and Choiniere (2024) went back to the original syntype series (a group of specimens used to bear a name) to see what could be salvaged.

M. readi, like many a species, was based on your standard pile o' bones of unclear association. Barrett and Choiniere (2024) regarded a subset, including several partial or complete vertebrae, an ulna, a radius, an ilium, a partial pubis, two tibiae, a fibula, and parts of four metatarsals, as representing one skeleton that they designated a lectotype (a name-bearing specimen selected from a syntype series). Furthermore, they were able to find several diagnostic features. It's not the prettiest type, but it *is* useable. The authors compared the lectotype to several other specimens frequently used as Melanorosaurus exemplars in the literature. Unfortunately, this part doesn't go quite so well for M. readi; NMQR 3314, one of the more important specimens (including a skull), is excluded on both anatomic and stratigraphic grounds. Another specimen, NMQR 1551, is considered consistent with M. readi but an assignment is not confirmed. (I personally hope it is, because I don't want an excuse for someone to try to bring back "Roccosaurus" as a name.) The upshot is M. readi is valid but not particularly helpful at the moment, pending reclamation of more complete material.

Plateosaurus

If Massospondylus is what you get when specimen assignment goes to the last species standing, Plateosaurus is what happens when people care too much. For sheer mind-numbing taxonomic complexity in the world of dinosaurs, it's hard to beat the adventures of Upper Triassic European prosauropods. An extensive historical review can be found in Regalado Fernández et al. (2023), along with detailed reports on many of the significant specimens involved in the story. One of the things I find useful is rather than synonymize the many lesser lights with a species of Plateosaurus, the authors treat them as dubious species. Why is this useful? Keeping them separate helps to keep the concept of the host species from drifting. It's not as if there's some prize for having the tidiest faunal list, after all; specimens should only be assigned to the level of confidence.

Stratigraphic distribution of German Plateosaurus and friends. I could quote every figure in this article, but then I might as well reproduce the whole thing. Figure 2 in Regalado Fernández et al. (2023) CC BY 4.0.

Coming at Plateosaurus from a different angle, Schaeffer (2024) redescribed the holotype (singular name-bearing specimen) of P. trossingensis, which became the type species of Plateosaurus a few years back (another long story). This gets right at the heart of the taxonomic drift: you have to establish your basis of comparison before you can compare anything to it. Continuing with this, Schaeffer ran a phylogenetic analysis of sauropodomorphs with Plateosaurus trossingensis based just on the holotype. Fortunately, the species behaved predictably in terms of general location and neighbors.

References

Barrett, P. M., and K. E. J. Chapelle. 2024. A brief history of Massospondylus: its discovery, historical taxonomy and redescription of the original syntype series. Palaeontologia africana 58: 97–131.

Barrett, P. M., and J. N. Choiniere. 2024. Melanorosaurus readi Haughton, 1924 (Dinosauria, Sauropodomorpha) from the Late Triassic of South Africa: osteology and designation of a lectotype. Journal of Vertebrate Paleontology 44(1): e2337802. doi: https://doi.org/10.1080/02724634.2024.2337802

Regalado Fernández, O. R., H. Stöhr, B. Kästle, and I. Werneburg. 2023. Diversity and taxonomy of the Late Triassic sauropodomorphs (Saurischia, Sauropodomorpha) stored in the Palaeontological Collection of Tübingen, Germany, historically referred to Plateosaurus. European Journal of Taxonomy 913(1): 1–88. doi: https://doi.org/10.5852/ejt.2023.913.2375

Schaeffer, J. 2024. Osteological redescription of the holotype of Plateosaurus trossingensis (Dinosauria: Sauropodomorpha) from the Upper Triassic of SW Germany and its phylogenetic implications. Journal of Systematic Palaeontology 22(1): 2335387. doi: https://doi.org/10.1080/14772019.2024.2335387

Catching up with NPS proboscideans

I had occasion recently to review some of the records in the 2020 National Park Service fossil proboscidean inventory (Mead et al. 2020), so I thought it would be useful to produce an update on the roster of parks, National Natural Landmarks, and National Historic Landmarks that we included at that time.

Secrets of "Secrets Of The Dinosaur Mummy", or: How I Got An IMDb Entry

A few days back I did something I hadn't done since 2008: I watched "Secrets of the Dinosaur Mummy". This was mostly because I had a friend over and wanted to show her what the Leonardo thing was all about, along with my minor claim to small-screen fame. Obviously going that road brought out the memories, so I thought maybe it was time to have a look back. Getting the "Truth in Advertising" out of the way up-front: I don't actually have a lot of secrets about "Secrets of the Dinosaur Mummy", but I do know a title when I see one!

It turned out to be unexpectedly difficult to get a good photo of the cover, being moderately glossy and dark, but you get the idea. One of these two blurbs is good for a chuckle, given the prominence of the speaker in the documentary.

Conard Fissure

It's been a while since we've done a good old-fashioned Pleistocene cave fauna, and there's one more wanting to complete the Big Three of Irvingtonian eastern North America. We've already spent some time with Cumberland Bone Cave and Port Kennedy Bone Cave, so now it's the turn of Conard Fissure in northwestern Arkansas.